1. Field of the Invention
The present invention relates to an optical disk apparatus equipped with a focus bias means to execute focus servo control by applying a focus bias from a focus bias generator circuit to a focus error signal in an operation of recording data on and/or reproducing the same from an optical disk such as a compact disk, a magneto-optical disk or the like used for data storage and so forth.
2. Description of the Related Art
In the optical disk apparatus known heretofore, it is customary to execute the following servo control.
For example, a light beam emitted from a light source is irradiated to the surface of an optical disk, and a return light beam from the optical disk is received by a photo detector.
A focus error signal and a tracking error signal are detected on the basis of the outputs of focus sensors which are divisions of the photo detector.
The focus error signal and the tracking error signal thus detected are supplied to a focus servo circuit and a tracking servo circuit respectively, thereby executing focus servo control and tracking servo control.
FIG. 1 shows an exemplary structure of such optical disk apparatus.
In FIG. 1, the optical disk apparatus 1 comprises an optical pickup 2, and a tracking servo circuit 5 and a focus servo circuit 6 which receive, via amplifiers 3 and 4 respectively, a tracking error signal and a focus error signal obtained on the basis of the difference between the light quantities of individual light receiving elements of a photo detector in the optical pickup 2.
The optical disk apparatus 1 further comprises a tracking driver 7 for driving an actuator of the optical pickup 2 under control on the basis of a servo signal from the tracking servo circuit 5 to thereby move an objective lens in the tracking direction, a focus driver 8 for driving the actuator of the optical pickup 2 under control on the basis of a servo signal from the focus servo circuit 6 to thereby move the objective lens in the focusing direction, and an adding circuit 9a for applying a focus bias, which is obtained from the focus bias generator circuit 9, to the focus error signal outputted from the amplifier 4.
The optical pickup 2 has a known structure wherein an objective lens (not shown) is held to be movable biaxially, so that when a tracking coil and a focus coil provided in the actuator are fed with current, the objective lens can be driven biaxially under control in both of the tracking and focusing directions.
The tracking servo circuit 6 is supplied with the tracking error signal obtained from the optical pickup 2 and amplified by the amplifier 3, and then outputs a tracking control signal to the driver 7 in accordance with the tracking error signal so as to minimize the tracking error.
The focus servo circuit 6 is supplied with the focus error signal obtained from the optical pickup 2 and amplified by the amplifier 4, and then outputs a focus control signal to the driver 8 in accordance with the focus error signal so as to minimize the focus error.
The driver 7 serves to drive the actuator of the optical pickup 2 in response to the tracking control signal received from the tracking servo circuit 5, thereby moving the objective lens in the tracking direction to minimize the tracking error.
Meanwhile the driver 8 serves to drive the actuator of the optical pickup 2 in response to the focus control signal received from the focus servo circuit 6, thereby moving the objective lens in the focusing direction to minimize the focus error.
Since the minimum point of the focus error signal may sometimes fail to coincide with the least jitter point of a reproduced signal, a focus bias obtained from the focus bias generator circuit 9 is applied to the focus error signal for causing the minimum point of the focus error signal to coincide with the least jitter point of a reproduced signal.
In the optical disk apparatus 1 of the structure mentioned above, the focus bias is adjusted in the following manner.
At the time of assembling the optical disk apparatus 1, focusing is performed in an on-state of the focus servo, and an adjusting rheostat 9b incorporated in the focus bias generator circuit 9 is manually operated while observing the RF signal from the optical pickup 2 and monitoring the value of the jitter, whereby an optimal focus bias is determined with respect to the individual optical disk device 1.
However, in the optical disk apparatus of the above structure, a tact time required for determining the focus bias is long with another disadvantage of necessitating the adjusting rheostat 9b, hence raising a problem of higher cost with regard to the component parts.
Further in an operation of recording data on and/or reproducing the same from an optical disk, the refractive index of the optical disk to an incident light beam is rendered different if the material of the disk is different, so that the incident light quantity of the return light beam to the photo detector may also be changed. In addition, occurrence of ambient temperature fluctuation brings about some harmful influence inclusive of a positional deviation of the objective lens due to the resultant temperature fluctuation in the apparatus.
Consequently, it becomes difficult to adjust the focus bias exactly to its optimal value in reproduction of data from the optical disk.